Structural, optical and thermal properties of V-doped GaN thin films grown by MOCVD technique

Thermochimica Acta 682 (2019) 178428

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Structural, optical and thermal properties of V-doped GaN thin films grown by MOCVD technique

T

M. Souissia, , T. Ghribb,c, A. Al-Otaibib, I.A. Al-Nuaimb, M. Bouzidid ⁎

a

Higher Institute of Computer Sciences and Communication Techniques of Hammam Sousse, Sousse 4011, Tunisia Department Physics, College of Science, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, 31441, City Dammam, Saudi Arabia c Nanomaterials Technology Unit, Basic and Applied Scientific Research Center (BASRC), College of Science of Dammam, Imam Abdulrahman Bin Faisal University, P. O. 383, Dammam 31113, Saudi Arabia d Laboratory of Physical of Materials (LPM), Faculty of Science, University of Monastir, Monastir 5000, Tunisia b

ARTICLE INFO

ABSTRACT

Keywords: GaN Metal Organic Chemical Vapor Deposition Characterization Heat transfer Vanadium

V-doped GaN epitaxial thin films were grown by Metal Organic Chemical Vapor Deposition (MOCVD) on c-plane sapphire substrate. Structure, surface morphology, gap energy, thermal conductivity, and thermal diffusivity are studied. The results show that the gap energy does not change with this V-doping concentration but its thermal conductivity decreases to a factor of about 3.5 from 128 to 36 W/mK. This behavior may be associated with the structural changes that are taking place with the addition of V content, showing that the material presents a good performance asked in certain applications.

1. Introduction Gallium nitride (GaN) has been widely studied as a potential material for various applications in optoelectronic devices, such as visible and ultraviolet (UV) light emitting diodes (LEDs) [1,2]. GaN has an important wide band gap (3.4 eV) semiconductor that holds promise in power electronic devices and is currently under study due to its remarkable applicability in response to its tunable properties. Electrical, magnetic, and optical properties of GaN are mostly size and surface dependent. Transition metal (TM)-doped GaN thin films can play an important role for a diversity of practical applications due to the charge and the spin of electrons that lead to a new magnetic, optical and transport properties of these materials. Impurities such transition metals create deep energy levels in the band gap (defect states) of III-V materials, which compensates the residual free charge carriers and increases resistivity [3–5].Therefore, it is important to behold the precise thermal conductivity values of the corresponding material and their reliance on doping concentrations in order to perform on the device design. The reliable thermal conductivity values are one of the most important property for material selection criteria in order to get the best performance of numerous high-power applications envisaged for GaN devices, such as lasers and power electronics, can be limited by the heat transfer through the GaN substrate and limiting the device performance and reliability. Furthermore, GaN devices can be manufactured on insulating

⁎

substrates providing a first heat sink without recourse to supplemental ones. The non-destructive photothermal deflection (PTD) technique [6–8] is one of the most extensively employed for carrying out the thermal and optical properties of materials [9,10]. In this paper, an investigation of the optical and thermal conductivity of GaN thin film doped with vanadium is reported. A flow of VCl4 equal to 4 sccm is selected because PL signal of this flow has shown a wide blue band at room temperature [11]. 2. Experimental GaN:V layer with a growth rate of 2.5 μm/h was grown on c-plane sapphire (Al2O3) substrate (ls = 500 μm thickness) at 1120 °C by atmospheric pressure MOCVD technique in a vertical reactor. In this experiment, Ammonia (NH3), Tri-methyl gallium ((CH3)3Ga), vanadium (IV) chloride (VCl4) and silane (SiH4) were used as precursors for nitrogen, gallium, vanadium and silicon, respectively. The growth procedure was done by using sapphire substrate which initially annealed in 2 slm of N2, 3 slm of NH3, and 2 slm of H2 at 1080 °C for 7 min. After nitridation step, a SiN treatment was carried out at the same temperature. Directly after SiN treatment, the temperature was decreased to 600 °C to grow a low-temperature (LT) GaN buffer layer. Finally, GaN:V took place at high temperature (HT)1120 °C by simultaneously introducing 10 sccm (CH3)3Ga (40 μmole/min) and VCl4 into the reactor, a GaN:V layer of lc = 2.5 μm thickness was grown and was

Corresponding author. E-mail address: [email protected] (M. Souissi).

https://doi.org/10.1016/j.tca.2019.178428 Received 13 January 2019; Received in revised form 11 September 2019; Accepted 29 September 2019 Available online 30 September 2019 0040-6031/ © 2019 Elsevier B.V. All rights reserved.

Thermochimica Acta 682 (2019) 178428

M. Souissi, et al.

measured in-situ with laser reflectometry [12]. Specific details of the experimental procedure and the growth conditions have been reported elsewhere [13,14]. The surface morphology of the specimens was observed with scanning electron microscopy (SEM) and atomic force microscopy (AFM). A High-Resolution X-ray Diffraction (HR-XRD) equipment was also used to characterize the crystal structure of HTGaN:V thin films. The optical absorption of the layers was recorded by using (Jasco V-570) spectrophotometer in the wavelength range from 350 to 375 nm. The thermal properties of the GaN and GaN:V thin films were determined by using the photothermal deflection (PTD) technique which is described widely. All measurements were carried out at ambient atmosphere and temperature (300 K).

n

dx = dy

dn dy dx

l /2 l /2

(3)

Where, l is the width of the pump beam spot on the specimen. By introducing the temperature of the fluid Tf, the Eq. (3) can be written: [10]

dx = dy

1 n

dn dTf dy dTf dx

l /2 l /2

(4)

The variation of the refractive index as a function of the temperature is in the order of 10−7 °C−1 in the air and 10-4 °C-1 in the liquid. As the rise of the temperature compared to the ambient one is a few degrees, the quantity dn will be considered constant, therefore the dedTf

flection ψ of the beam is written in the case of a uniform heating in the form:

3. Photothermal deflection technique This technique consists in heating the specimen with a modulated light beam of intensity I = I0 (1 + cos t ) that will be absorbed on the surface and generates a thermal wave which contains in its side all thermal properties of the heated material. This thermal wave will propagate in the specimen and in the surrounding fluid (the air in our case) and will induce a temperature and a refractive index gradient in the fluid. The fluid index gradient will cause a deflection ψ of a (He-Ne) laser probe beam skimming the specimen surface at a distance x. This deflection may be related to the thermal properties of the specimen. The specimen was heated by a halogen lamp light of 100 W power modulated by using a mechanical chopper at a variable frequency f as shown in Fig. 1. The laser probe beam path follows optical ray equation of the form [14]:

d dr (n )= ds ds

grad n

=

dn dx

(5)

If it designate by T0 the temperature at the specimen surface, a complex number which can be expressed as T0 = |T0 | ei where T0 and θ are its module and its argument of T0. The temperature in the fluid at a distance x from the specimen surface is:

Tf = T0 e

f xe j t

with f

= (1 + j)

f / Df

Thus, the complex deflection

=

(1)

dx 2 L dn = |T0 | e dy nµf dTf = | | e j(

Where, n is the refractive index of the medium, s is the curvilinear abscissa of the light beam along its trajectory and r is the position vector of a point M of this trajectory. As the angle Ψ is weak, the indy finitesimal curvilinear displacement will be: ds = (dx )2 + (dy ) 2 Therefore, a simplification of the optical ray equation will be as follows:

d dx (n ) = dy dy

l dn dTf n dTf dx

dx = dy

5 x µf e j ( + 4

t+ )

of the probe beam was deduced as: x µf ) e j t

(6)

Where, µ f = Df / f is the thermal diffusion length in the fluid, | | and represent respectively the amplitude and phase of the deflected probe beam, given as:

| |=

(2)

=

Therefore

2 l dn |T0 | e nµf dTf

x + µf

+

5 4

x µf

(7) (8)

Fig. 1. Experimental set-up used for PTD investigation: 1-Table of micrometric displacement, 2-Specimen, 3-Photodetector position, 4-Fixed laser source, 5-Halogen lamp, 6-Look-in amplifier, 7-Mechanical chopper, 8-Computer. 2

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M. Souissi, et al.

Fig. 2. Deflection of a probe beam passing through a graded index area in the fluid nearby the heated specimen.

Before the calculation of the probe beam deflection, the expression of the surface temperature T0 should be known. Then, it is mandatory to define the system and their compositions which are in our case two media; the substrate (sapphire) and the V-doped GaN epitaxial thin film. And all the system will be constituted of an assembly of four media (Fig. 2); the backing fluid which is the air in our case denoted (b), the sapphire substrate denoted (s), the deposited GaN: V thin film denoted (c) and the air surrounding the specimen denoted (f). Thereafter, the heat equations in these media are written as follows: 2T f x2

=

1 Tf for0 Df t

2T c x2

=

1 Tc Dc t

2T s x2

=

1 Ts for lc Ds t

ls

x

2T b x2

=

1 Tb for lc Db t

ls

lb

x

lf

2(1 + (1 (1

x

0

(11)

x

lc

ls

f xe j t

for0

Tc (x ) = (Ue

sx

Ts (x ) = (Xe

s (x + l s )

Tb (x ) = We

b (x + l c + l s ) e j t

+ Ve

sx

+ Ye

x

lf t

s (x + ls ) ) e j t

for lc

ls

for lc

x

0

for lc

ls

x

lb

x

lc

(15) (16)

ls

Where U, V, X, Y and E are complex numbers. Also, the heat flux in each medium will be after taking r = / c : f

= Kf

f T0 e

f xe j t

for 0

cx

Ve

c (x )

=

Kc

c (Ue

s (x )

=

Ks s (Xe

b (x )

=

Kb b We

s (x + l c )

x cx

Ye

b (x + l s l c ) e j t

rEe x ) e j s (x + l c ) ) e j t

for

lc

t

for lc for

ls

x

lc lb

ls x

x lc

ls ls

+ g )(1

c) e

c lc

+ (1

g )(1 + c ) e

c l c ]]

c) e

s ls

2(1

rc ) e

ls ]/

s ls ]

Fig. 3 shows the secondary ion mass spectroscopy (SIMS) profile of a multilayer structure consisting of alternating undoped GaN and GaN:V for different V-doping levels (2, 4, 6 and 10 sccm of VCl4). The turn on and off of the V dopant is respectively accompanied by an increase and a decrease of the V SIMS profile. This indicates V incorporation in GaN. The amount of V incorporation increases with the increasing of the VCl4 flow rate (insert Fig. 3). In this work, the doping of GaN has been taken with a VCl4 flow rate equal to 4 sccm. To investigate the crystal structure of the GaN:V thin films, HR-XRD measurement in 2θ-ω mode was carried out. Fig. 4 presents typical HR-XRD of the doped layer. The observation planes (0002) at 34.4 degrees and (0004) at 72.8 degrees corresponding to the GaN, showing that the GaN: V layer has a

(18)

0

+ g )(1 + c ) e

c lc ]

4. Results and discussion

(17)

lf

s l s [(1

+ b) e

c lc

From these equations it is denoted that only the thermal conductivity and thermal diffusivity of the backing and fluid which are the air are known having the thermal conductivity Kb = Kf = 0.022 W.m−1. K−1 and Db = Df = 20 mm2.s−1 which will be introduced with the values of lc = 2.5 μm and ls = 500 μm and the other unknown properties can be determined experimentally by varying the modulation frequency f and measuring the amplitude and phase of the photothermal signal and by comparing them with the theoretical Eqs. (7) and (8) while taking into consideration the Eqs. (21) or (22).

(14)

ls

b) e

c) e

c) e

c lc

(22)

(13)

Ee x ) e j

g )(1

s ls [(1

T0 = E [(1 r )(1 + c ) e s ls + (1 + r )(1 [(1 + g )(1 + c ) e s ls + (1 g )(1 c ) e

(12)

Because of the modulation of the pump beam, the solutions of these heat equations will be periodic and can be simplified as follows after taking i = (1 + i) f / Di :

Tf (x ) = T0 e

rc ) e

lc ]]/[(1

(21)

(10)

ls

r )(1 c ) e c lc + (1 + r )(1 + c ) e c lc (1 + b) e s ls [(1 r )(1 + c ) e c lc + (1 + r )(1

Whose E = A/( 2 2i/ µc2 ) can be obtained by replacing the Ts expression in the differential equation (eq. 11), b = Kb b/ Ks s , c = K c c /Ks s , g = Kf f / K c c , α the total optical absorption and µc = Dc / f is the thermal diffusion length in the deposited GaN or GaN:V thin film. To determine the thermal properties such as the thermal conductivity and thermal diffusivity of the deposited thin film, it is mandatory to know those of the substrate Al2O3, then the used model will be that of three media (backing/substrate/fluid) and not that of four media (backing/substrate/thin film/fluid) and the Eq. (20) will be simplified as:

(9)

Ae x (1 + e j t ) for lc

s l s [(1

T0 = E [(1 b) e 2(1 + rc ) e lc ]

(19) (20)

By writing the continuity of the temperature and the heat flux in each interface between media in x = 0, x = -lc and x = -lc-ls, the unknown parameters can be determined and the surface temperature takes the following relationship: 3

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M. Souissi, et al.

Fig. 3. SIMS profile of V in sandwiched structure consisting of alternating undoped GaN and GaN:V for different V doping levels (2, 4, 6 and 10 sccm of VCl4). Inset shows the evolution of SIMS intensity versus VCl4 flow rate.

Fig. 4. HXRD spectrum obtained for V-doped GaN layer grown with 4 sccm of VCl4 flow rate.

hexagonal symmetry. It has not observed any binary or ternary phases in the layer, such as GaVN or V-N compounds or V-Ga alloys. For Crdoped GaN layers, Suemasu et al [15] have been observed a ternary phase (GaCrN) in the layer. Scanning electron microscopy was used to investigate the surface morphology of the grown specimens. Fig. 5 shows top views of scanning electron micrographs of the undoped GaN and V-doped GaN thin films grown under H2 for 4 sccm VCl4 flow rate [16]. In the case of the undoped GaN layers, the surface is smooth (Fig. 5(a)). After doping the morphology of GaN films changes shown in (Fig. 5(b)), dense hexagonal GaN structures are observed. The surface of the specimen reveals zones not coalesced and consists of ridges and valleys. The degradation behavior of the surface may be due to the chlorine-induced back-etching from vanadium tetrachloride used for doping or due to high-vanadium surface contamination [17]. Fig. 6 shows AFM analysis

Fig. 5. SEM image obtained for GaN (a) and (b) V-doped GaN layer grown with 4 sccm of VCl4 flow rate.

4

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M. Souissi, et al.

Fig. 6. AFM image obtained for V-doped GaN layer grown with 4 sccm of VCl4 flow rate.

Fig. 7. Variation of (αE)2 versus energy E of GaN: V specimen. Dashed line is the extrapolation of a linear part of the curve of (αE)2 versus E to αE = 0.

of V-doped GaN layer. The image is taken in an area of 5 μm x 5μm. The surface of the layer presents a terrace-like morphology and indicates that the surface is not smooth. The dislocation density at about 108 cm−2 [14]. To determine the energy gap of the film, it was measured UV–vis absorbance spectrum which will be transformed as in Fig. 7 which shows the variation of (αE)2 versus photon energy E of the GaN:V thin films. The extrapolation of a linear part of the curve of (αE)2 versus E to the (αE)2 = 0 as shown in Fig. 7 (dashed line) is referred to a direct band gap energy (Eg) of the film. Thereby, the obtained gap energy value is Eg =3.396 eV. This obtained value is in good agreement with that of the pure GaN obtained by H. Amano et al. [18] who deposited GaN by MOCVD technique and obtained 3.4 eV. Thus, it can be deduced that the doping of GaN with this percentage of vanadium cannot change remarkably its gap energy and also its electrical conduction behavior. In order to determine the thermal properties of the pure GaN and V doped GaN thin films, the variation of the photothermal signal with the square root of the modulation frequency was studied. Thermal

conductivity K and the thermal diffusivity D of these films are determined by comparing the experimental phase and amplitude curves to the corresponding theoretical ones. Fig. 8 represents the experimental and theoretical variations of the amplitude and the phase of photothermal signal according to the square root of the modulation frequency, for a sapphire substrate (Al2O3), GaN grown on Al2O3 (GaN/ Al2O3) and V-doped GaN grown on Al2O3 (GaN:V/Al2O3). The best theoretical fitting between the experimental and theoretical curves gives the values of the couple (K, D). The thermal conductivity of GaN at room temperature was found 128 W/mK which is comparable to the first experimental measurements of the thermal conductivity K of GaN specimens elaborated by hydride vapor phase epitaxy that revealed a value of about 130 W/mK at 300 K [19]. Florescu et al. [20,21], using the scanning thermal microscopy technique, determined that the thermal conductivity for the high-quality crystals of the GaN layers elaborated by the lateral epitaxial overgrowth (LEO) is about 170–180 W/mK. After doping, the value of the thermal conductivity decreases and reaches 36 W/mK. The relatively large difference 5

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M. Souissi, et al.

Fig. 8. Experimental and theoretical amplitude and phase signals versus the square root modulation frequency of the (Al2O3), GaN grown on Al2O3 (GaN/ Al2O3) and V-doped GaN grown on Al2O3 (GaN:V/ Al2O3).

between these two values before (128 W/mK) and after doping (36 W/ mK) of the thermal conductivity can be attributed to the changes in the internal structure of GaN:V thin films reported elsewhere [16,14] which cannot be deduced with other techniques and it will decrease when there are some crystal defects or dislocations which are in direct relationship to the propagation of phonons along the specimen [22]. This point can be confirmed by the measurement of the thermal diffusivities. In fact, for Al2O3, GaN/Al2O3 and GaN:V/Al2O3 the measured thermal diffusivities are 30, 82 and 16 mm2/s, respectively. It is denoted that it decreases with V doping. On the other hand, a kinetic study shows that the thermal diffusivity D of a material is directly related to the mean free path λ by the following equation:

=

3D v

photothermal deflection (PTD) technique. This study has shown that despite the gap energy and the crystalline structure that are not changed with doping effect, the thermal properties know a most important decreasing which can be related to the insertion of the vanadium in the GaN crystalline lattice which plays the role of thermal nodes leading to an overall thermal insulation. References [1] S. Nakamura, M. Senoh, T. Mukai, High-power lnGaN/GaN double-heterostructure violet light emitting diodes, Appl. Phys. Lett. 62 (1993) 2390. [2] M. Asif Khan, J. Kuznia, D. Olson, W. Schaff, J. Burm, M. Shur, Microwave performance of a 0.25 μm gate AlGaN/GaN heterostructure field effect transistor, Appl. Phys. Lett. 65 (1994) 1121. [3] M. Bonnet, J.P. Duchemin, A.M. Huber, G. Morillot, G.J. Rees (Ed.), Semi-Insulating III-V Materials, Not-tingham 1980, Shiva, London, 1980, p. 68. [4] A.T. Macrander, J.A. Long, V.G. Riggs, A.F. Bloemeke, W.O. Johnson Jr, Electrical characterization of Fe‐doped semi‐insulating InP grown by metalorganic chemical vapor deposition, Appl. Phys. Lett. 45 (1984) 1297. [5] H.M.D. Hobgood, R.C. Glass, G. Augustine, R.H. Hopkins, J. Jenny, M. Skowronski, W.C. Mitchel, M. Roth, Semi‐insulating 6H–SiC grown by physical vapor transport, Appl. Phys. Lett. 66 (1995) 1364. [6] N. Yacoubi, H. Mani, Photoacoustic and Photothermal, Phenomena II, 62, Springer series in optical sciences, Heidelberg, 1990, pp. 173–177. [7] P.K. Kuo, M.J. Lin, C.B. Reyes, L.D. Favro, R.L. Thomas, D.S. Kim, S.-Y.I. Zhang, L.J. Inglehart, D. Fournier, A.C. Boccara, L.J. Inglehart, N. Yacoubi, Mirage-effect measurement of thermal diffusivity, I: experiment, Can. J. Phys. 64 (1986) 1168–1175. [8] A. Salazar, A. Sanchez-lavega, J. Fernandez, Thermal diffusivity measurements in solids by the ``mirage’’ technique: experimental results, J. Appl. Phys. 69 (1991) 1216–1223. [9] J. Bodzenta, B. Burak, W. Hofman, M. Gala, J. Kucytowski, T. Lukasiewicz, M. Pyka, K. Wokulska, Influence of metallic and lanthanide dopants on the thermal diffusivity of lithium niobate crystals, J. de Physique IV France 117 (2004) 7–12. [10] Taher Ghrib, Noureddine Yacoubi, Faycel Saadallah, Simultaneous determination of thermal conductivity and diffusivity of solid samples using the “Mirage effect” method, J. Sens. Actuators A 135 (2007) 346–354. [11] M. Souissi, Z. Chine, A. Bchetnia, H. Touati, B. El Jani, Photoluminescence of V-doped GaN thin films grown by MOVPE technique, Microelectron. J. 37 (2006) 1–4. [12] Y. Raffle, R. Kuszelewicz, R. Azoulay, G. Le Roux, J.C. Michel, L. Dugrand, E. Toussaere, In-situ thickness measurement of MOVPE grown GaAs GaAlAs by laser refleetometry, Microelectron. Eng. 25 (1994) 229–234. [13] Z. Benzarti, I. Halidou, T. Boufaden, B. El Jani, S. Juillaguet, M. Ramonda, Effect of SiN treatment on GaN epilayer quality, Phys. Status Solidi 1 (2004) 7. [14] M. Souissi, A. Bchetnia, B. El Jani, Growth of vanadium-doped GaN by MOVPE, J. Cryst. Growth 277 (2005) 57. [15] T. Suemasu, K. Yamaguchi, H. Tomioka, F. Hasegawa, Room‐temperature ferromagnetism in Cr‐doped GaN films grown by MOMBE on GaAs(111)A substrates, Phys. Status Solidi (C) 7 (2003) 2860. [16] M. Souissi, H. Touati, A. Fouzri, A. Bchetnia, B. El Jani, Effect of carrier gas on the surface morphology of V-doped GaN layers, Microelectron. J. 39 (2008) 1521–1524. [17] A. Rebey, A. Bchetnia, C. Benjeddou, B. El Jani, P. Gibart, New vanadium dopant

(23)

Where v is the heat propagation velocity inside the material. The mean free path of the particles conductors of the heat such as electrons and/ or phonons is sensitive to grain size and the crystalline parameters. These dependencies are summarized in the following equation:

1

=

1 G

+

1

+

r

1 D

(24)

Where λG is the contribution due to the grain sizes, λr is the contribution due to the crystal lattice size and λD is the contribution due to volume distortion which can be deduced from XRD spectrum, but from the Fig. 4 it doesn’t denote any new peaks due to the vanadium doping which leads to the interpretation that the mixture gives a substitutional alloy by changing atoms from the GaN crystal lattice by those of vanadium. This influence highly the thermal propagation according to the nature of the added atom which can play in our case the role of heat conduction nodes. These properties of the conservation its electrical, optical and structural properties and the decreasing of its thermal properties gives to the GaN:V thin film a good importance in certain devices that need these special behaviors. 5. Conclusion The thermal properties of GaN and V-doped GaN thin films grown on sapphire substrate with MOCVD technique were studied with

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M. Souissi, et al. precursor for GaAs growth by metalorganic vapor-phase epitaxy, J. Cryst. Growth 194 (1998) 292. [18] H. Amano, N. Sawaki, I. Akasaki, Y. Toyoda, Metalorganic vapor phase epitaxial growth of a high quality GaN film using an AlN buffer layer, Appl. Phys. Lett. 48 (1986) 353. [19] E.K. Sichel, J.I. Pankove, Thermal conductivity of GaN.25–360K, J. Phys. Chem. Solids 38 (1997) 330. [20] D.I. Florescu, V.M. Asnin, F.H. Pollak, A.M. Jones, J.C. Ramer, M.J. Schurman,

I. Ferguson, Thermal conductivity of fully and partially coalesced lateral epitaxial overgrown GaN/sapphire (0001) by scanning thermal microscopy, Appl. Phys. Lett. 77 (2000) 1464. [21] D.I. Florescu, V.M. Asnin, F.H. Pollak, Thermal conductivity measurements of GaN and AlN, Compound Semicond. 7 (2001) 62. [22] D. Kotchetkov, J. Zou, A.A. Balandin, D.I. Florescu, F.H. Pollak, Effect of dislocations on thermal conductivity of GaN layers, Appl. Phys. Lett. 79 (2001) 4316.

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